1. Field of the Invention
This invention relates to a spin coating method which is extensively employed in the field of manufacture of semiconductor devices, and to a spin coating device for practicing the method.
2. Background of the Invention
Heretofore, in the field of manufacture of semiconductor devices, a coating solution of photosensitive material such as photo-resist or a gelatinous silver halide emulsion is applied by a spin coating method to a substrate, such as a silicon wafer or photo mask blank made up of a glass plate with a metal film.
A spin coating device for practicing the spin coating method, as shown in FIG. 2, comprises a turn table 4 on which is placed a substrate 1 to be coated with the coating solution. A vertical rotary shaft 5 rotates the turn table 4. An electric motor 6 rotates the vertical rotary shaft 5.
The spin coating device is operated as follows. A solution supplying nozzle 3 for supplying the coating solution is set above the center of the turn table 4.
After the substrate 1 is placed on the turn table 4, a predetermined amount of coating solution 2 is supplied onto the substrate 1 through the nozzle 3. Thereafter, the turn table 4 is rotated at low speed until the coating solution 2 spreads over the entire surface of the substrate 1. In succession to this operation, the turn table 4 is rotated at high speed so that a layer of coating solution 2 is formed on the substrate 1 to a desired thickness.
In the case where the substrate 1 is relatively small in size (for instance, three or four inches maximum), or a kind of photoresist coating solution is used, the above-described method has no problem at all.
However, it has been found that, in the case where the substrate is relatively large, at least five or six inches, or another king of coating solution is used, it is impossible to form a layer of coating solution of uniform in thickness by the above described method.
For instance, in the case where the coating solution 2 is a silver halide emulsion with a gelatin binder, the film formed on the central portion of the substrate where the coating solution is first supplied is clearly different in thickness from the film which is formed on the other portion, namely, the peripheral portion, after the coating solution 2 is spread by rotation of the turn table 4.
This phenomenon is significant especially in the application of a silver halide emulsion onto the substrate. That is, when the substrate is not sufficiently wetted by the coating solution, the film formed on the substrate is not uniform in thickness. The portions of the film which are different in thickness are different also in sensitivity and in developing speed, and therefore the resultant image is not uniform in line width. Accordingly, the characteristic of a semiconductor device formed using the image is different from the desired one.
In order to eliminate the above-described difficulties, Japanese Patent Application Publication No. 13377/1981 has disclosed a method in which a coating solution of gelatinous silver halide emulsion is substantially uniformly applied to the substantially entire surface of a substrate held horizontally and then the substrate is rotated. More specifically, in the method, the coating solution is spread manually or automatically. In the manual method, the coating solution is manually spread over the entire surface of the substrate with a bar. In the automatic method, a number of coating-solution supplying nozzles are juxtaposed along one side of the substrate. The nozzles thus arranged are moved towards the opposite side so that the entire surface of the substrate is coated with the coating solution. Alternatively, a number of coating-solution supplying nozzles are arranged over the substrate to simultaneously coat it with the coating solution. In a still further automatic method, one coating-solution supplying nozzle is set above the substrate, and it is moved over the substrate while applying the coating solution through it.
In order to coat the substrate uniformly, it is necessary to cover about 50% to 80% of the area of the substrate with the supplied coating solution. It takes a relatively long period of time to cover the area with a single nozzle. Therefore, it is necessary to arrange a number of solution coating nozzles over the substrate, or to move a solution coating nozzle over it. That is, the coating device needs a relatively intricate mechanism to quickly cover about 50% to 80% of the area of the substrate with the coating solution. Especially in the case where the coating solution is a gelatin silver halide emulsion, it is not smoothly spread over the substrate. In order to allow a coating solution of this type to smoothly spread over the substrate, the coating solution must be supplied through the solution supplying nozzle more than is actually required. That is, the coating solution is uneconomically used.